In delta-sigma modulators in high precision delta-sigma analog-to-digital converters an analog input signal is summed with a reference voltage at a summing node to provide an analog difference signal. The analog difference signal is integrated and quantized to produce a digital output. The digital output is also coupled to a digital-to-analog (D/A) converter which is used to select which reference level (e.g., +V.sub.R and -V.sub.R) is to be applied to the summing node in the next sampling period. FIG. 1A is a schematic diagram of such a converter. The D/A can also be used in a delta-sigma digital-to-analog converter as shown in FIG. 1B. The accuracy of the A/D or D/A conversion is dependent upon the accuracy of the reference level.
In an integrated circuit delta-sigma modulator the reference voltage sources do not have zero impedance since it is impractical to produce reference voltages with virtually zero source impedance. Moreover, in some circuits an antialias network may be used to filter high frequency noise from the reference voltage. If the loading on the reference voltage is a function of the analog input signal then the delta-sigma modulator will have non-linear errors due to this signal-dependent loading of the reference voltages.
Therefore it can be appreciated that a delta-sigma modulator which provides data independent loading of a reference is highly desirable.